#include <cctype>
#include<stdio.h>
#include <iostream>
using namespace std;

class AlphaSet
{
    private:
        unsigned bin;
    public:
        AlphaSet() { bin = 0 ;};
        AlphaSet(char*);
        AlphaSet operator*(AlphaSet&);
        AlphaSet& operator~();
        friend ostream& operator<<(ostream&, AlphaSet&);
};

AlphaSet::AlphaSet(char* s)
{
    bin = 0;
    while (*s)
    {
        bin |= (1 << (tolower(*s) - 'a'));
        s++;
    }
};

AlphaSet AlphaSet::operator*(AlphaSet& set)
{
    AlphaSet result;
    result.bin = bin & set.bin;
    return result;
};

ostream& operator<<(ostream& out, AlphaSet& set)
{
    unsigned bit = 1;
    int i = 0;

    for (i; i < 26; i++)
    {
        if ((set.bin & bit) > 0)
            out << (char)('a' + i);
        bit = bit << 1;
    }

    return out;
}
AlphaSet& AlphaSet::operator ~(){
    bin = ~bin;
    return (*this);
}
int main(int argc, char* argv[])
{
    if (argc != 3)
        return 1;

    AlphaSet x(argv[1]);
    AlphaSet y(argv[2]);
    AlphaSet z;

    z = (~x) * (~y);
    cout << z << endl;
    return 0;
};